I know this is an old thread but perhaps I can help clarify this for other people who stumble onto this site. I'm going to oversimplify and leave out some important details but they would just confuse most people.
To start, you have to recognize that everything on every digital phone is data. The voice is data. SMS is data. MMS is data. Internet is data. What your wireless provider is doing is essentially the same thing that your cable company does. Each one of these things is a "service" and each service either uses a separate frequency or a separate tag on the packets of data to route the traffic over the networks specifically dedicated for each of those services. SMS itself utilizes the control channel of the cell signal to a phone in much the same way that caller ID information is sent down the D channel on a PRI. That control channel is there to handoff your phone from cell to cell as you move, to setup and take down calls by assigning the frequencies to each phone as needed, and a few other things. When the control channel is in use for one reason no other data can traverse that channel.
The resources for sms are fairly limited It was never expected to be utilized as it is today. The reason text messaging used to be so expensive was because of the finite capacity for the traffic. They also managed to find a way to send small multi-media files over this same transport but it was even more expensive.
Fortunately, as smart phones came out with internet capability and everyone started getting data plans, (often FORCED to get a data plan), carriers were able to setup phones to do a form of "highest efficiency routing" where the phone would decide if it would be cheaper for the company and less burdensome on the network to send a message via traditional internet or if it should use the true SMS/MMS channels to send the message. Any more, the majority of sms and mms is sent and received over the customer's data plan to save the control channels from congestion. With all the users out there, if everyone was using the control channels for their messaging, it would be like new years every day and no sms messages would make it for days!
Despite that being the preferred routing of SMS/MMS, it will still utilize the control channels to send and receive if there is no "data" connection available. (I don't like the word "data" in place of "internet". As I stated before, the control channel, SMS, MMS, GPRS, Edge, CDMA, GMS, PCS, etc are all data connections ever since voice went to digital. But some marketing genius decided to call it "data" plans because when it first came out, WAP wasn't actually capable of handling enough throughput to call it a real internet connection. So if you have a text and MMS plan, you can send them fine without a "data plan". They will just default to the old standard of using the control channels. If you leave your data turned on you phone and you don't have a data plan, then SMS and MMS will route to control channels while everything else will simply not be able to connect to the internet since there is no bridge connecting the control channel network to the public internet. Your apps will attempt to reach out to the public internet but the edge routers will refuse access to the internet. The apps can't access the control channels because the control channel network doesn't use TCP/IP for transport.
The result is this.... If you have a smart phone and your data is turned on and you have no data plan, you can send and receive SMS and MMS. If you turn off the data connection on the phone, you will no longer be able to send SMS or MMS but since it's part of the control panel, you'll still receive SMS and MMS. If you have a data plan and it is turned on, your phone will choose to send SMS and MMS over the public internet via a process similar to a VPN which rides the internet to the closet tower and then if the person has data, will use data to send to them. And if not, will merge onto the control channel via switching to get to the phone.
Hope that all makes sense. I know I left out some details and took some liberties to be slightly inaccurate at times for the sake of brevity. It was long enough as it is. lol